There is a convenient package to travel to Earth Celebration:

EC Charter Bus (Overnight Buses from Tokyo)
The EC Charter Bus provides transportation directly from Tokyo to Earth Celebration. The bus leaves the city at night and arrives in Ogi the following morning, so you can enjoy the festival from the very beginning. This package offers considerable savings of both time and money. There is a return bus service departing from Ogi Port at 22:30 on the last ferry after the Shiroyama Concert on Aug. 24 (Sun).